The present invention relates to variably controlling the opening/closing timing of an intake valve and an exhaust valve of an internal combustion engine, and especially relates a technique for reducing the HC emission at the engine cold.
At the engine cold, since a large amount of unburned HC is included in the exhaust due to the low combustion temperature, and also the exhaust purifying catalyst is not yet activated due to the low exhaust temperature, an HC emission amount is likely to be increased. Especially at the end of exhaust stroke, the unburned HC adhered to an inner wall of cylinder is peeled off to be discharged, which leads to a further increase in the HC emission amount from the cylinder.
According to Japanese Unexamined Patent Publication No. 2000-8896, a variable valve mechanism is equipped to an intake valve, and at the engine cold, the opening timing of the intake valve is controlled to an advance angle so as to increase a valve overlap amount. Accordingly, the HC of small molecular weight generated around the top dead center is sucked back to an intake system, and an HC amount residing within the cylinder is increased, so as to be combusted by a next stroke, thereby achieving the reduction of the emission of unburned HC.
However, when the opening timing of the intake valve is advanced and the valve overlap amount is increased, since the combustion temperature is too much reduced due to an increase in the internal EGR, there is a possibility of deterioration of operating performance.
Especially, in a lean combustion by a lean mixture in a specific operation region, that is, in a lean combustion at the engine cold where the engine is operated the lean mixture, the combustion temperature is reduced by the internal EGR even the normal setting of the valve overlap amount, so that a large amount of unburned HC exists inside the cylinder to deteriorate the combustion performance. As a result, the amount of HC discharged from the cylinder is increased.
Moreover, since the exhaust purifying catalyst is not activated, the purifying efficiency of HC is degraded, and it is difficult to reduce the HC effectively.
The present invention has been achieved in view of the aforementioned problems in the conventional technique and an object of the invention is to reduce the HC emission at the engine cold, without deteriorating the operating performance of an engine.
In order to achieve the above object, according to the preset invention, the valve timing of an intake valve and an exhaust valve is controlled as follows.
It is detected that an engine is in a cold state, and if an air-fuel ratio is set to be lean at the engine cold, the opening timing of the exhaust valve is controlled to a retard angle side with respect to a reference valve timing set based on operating conditions, such as, an engine load, an engine rotation speed and the like, while reducing a valve overlap amount where the opening timing of the intake valve and the opening timing of the exhaust valve overlap with each other,.
According to such a constitution, the opening timing of the exhaust valve is retarded to accelerate the oxidation of unburned HC in an expansion stroke, while reducing an amount of the internal EGR (burned gas amount) to secure a good engine combustion, thereby reducing the HC emission.
The other objects and features of the present invention will become understood from the following description with the accompanying drawings.